WordPress at a glance
filter is not described

attachment_fields_to_save filter-hook . WP 1.0

This filter is documented in wp-admin/includes/media.php

Usage

add_filter( 'attachment_fields_to_save', 'filter_function_name_5270', 10, 2 );
function filter_function_name_5270( $post, $attachment_data ){
	// filter...

	return $post;
}
$post
-
$attachment_data
-

Where the hook is called

wp_ajax_save_attachment_compat()
attachment_fields_to_save
edit_post()
attachment_fields_to_save
media_upload_form_handler()
attachment_fields_to_save
wp-admin/includes/ajax-actions.php 3120
$post = apply_filters( 'attachment_fields_to_save', $post, $attachment_data );
wp-admin/includes/post.php 401
$translated = apply_filters( 'attachment_fields_to_save', $translated, $attachment_data );
wp-admin/includes/media.php 762
$post = apply_filters( 'attachment_fields_to_save', $post, $attachment );

Where in WP core the hook is used WordPress

wp-admin/includes/ajax-actions.php 34
add_filter( 'attachment_fields_to_save', 'image_attachment_fields_to_save', 10, 2 );